The present invention relates to a transmitting apparatus and a receiving apparatus of orthogonal frequency division multiplexing (OFDM) modulated signals, a communication system comprising a transmitting apparatus and a receiving apparatus, and further a signal processing method in each of the transmitting apparatus, receiving apparatus, and the communication system.
As a system of transmitting multiplexed channels using multi-carrier communication, a digital audio broadcasting (DAB) system has already been put into practical use in Europe. In the DAB system, OFDM is used as the modulation method. In a broadcasting system using the OFDM modulation method, a plurality of orthogonal sub-carriers modulated by quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) or another data modulation method are multiplexed to produce an OFDM modulated signal. By providing a guard interval which is made by recurring part of an effective symbol waveform in a valid symbol period of the OFDM signal, the influence of multi-path error (ghost wave) in a radio wave transmission of a ground wave can be reduced. Namely, by making the guard interval longer than the expected delay time of the ghost wave, ghost interference can be easily removed at the reception side.
FIG. 22 and FIG. 23 show an example of a wireless communication system using the OFDM modulation method, in which FIG. 22 shows a configuration of a transmitting apparatus using the OFDM modulation method, and FIG. 23 shows the configuration of a receiving apparatus for receiving an OFDM modulated signal.
As shown in FIG. 22, a transmitting apparatus using the OFDM modulation method is configured by a channel encoder 101, a time interleave circuit 102, a symbol mapping circuit 103, a multiplex circuit (MUX) 104, a frequency interleave circuit 105, a differential modulation circuit 106, an inverse fast Fourier transform circuit (IFFT) 107, and a transmitter (Tx) 108. The channel encoder 101 encodes an input data bit train (bit stream) DBSM of an M-th channel. Note that the related encoding includes, for example, error correction and encoding. The order of the encoded data train is switched at random on the time axis by the time interleave circuit 102. Time interleaving is a method for coping with so-called burst noise, that is, a large amount of noise generated in a transmission path concentrated in a certain constant time band. Time interleaving is carried out with respect to a data series to be transmitted on the transmission side, while deinterleaving is carried out on the reception side to return the received data series to the original order. For this reason, when burst noise is generated, the influence of the noise is dispersed in the transmission signal and complete interruption of data transmission can be prevented.
The time interleaved data is mapped by the symbol mapping circuit 103 with respect to each sub-carrier in accordance with a predetermined data modulation method. Note that the data modulation method used for the mapping may be of various types such as the QPSK, 8PSK, and the 16QAM. Typically QPSK is used in DAB. The symbol mapping circuit 104 creates a symbol stream corresponding to the input data series.
The mapped symbol stream consisting of the M channels is input together with the symbol streams of other channels created by similar processing to the multiplex circuit 104. Only inputs DBS 1 and DBS 2 are shown. The stream is then multiplexed by the multiplex circuit 104. As the simplest example, the multiplex circuit 104 can realize multiplexing by simply connecting the symbol streams of the plurality of channels in series. The multiplexed symbol streams are rearranged by the frequency interleave circuit 105, and the differential modulation circuit 106 differentially modulates each symbol with the respective symbol transmitted one modulation period before.
The differentially modulated symbol streams are converted to parallel data by a serial/parallel conversion circuit, not shown. This parallel data becomes the modulated data in each sub-carrier and can be regarded as a vector of a spectrum on a frequency axis. The modulated data is transformed to a transmission signal on the time axis by the inverse fast Fourier transform circuit 107, modulated to a high transmission frequency by the transmitter 108, and radiated to space via an antenna 109.
On the reception side, reverse processing to that on the transmission side is carried out to demodulate the received OFDM modulated wave and thereby to reproduce the original information data streams. As shown in FIG. 23, the receiving apparatus is configured by a channel decoder 111, a time deinterleave circuit 112, a bit extraction circuit 113, a channel selection circuit 114, a frequency deinterleave circuit 115, a differential demodulation circuit 116, a fast Fourier transform circuit (FFT) 117, and a receiver (Rx) 118. The receiver 118 receives a signal of an intended frequency band in the high frequency reception signal excited at a reception antenna 119. The received signal is converted to a baseband signal by frequency conversion. The baseband signal is Fourier transformed by the fast Fourier transform circuit 117. As a result, the received symbols corresponding to the modulated data of the sub-carriers on the frequency axis are found.
Each received symbol fluctuates in phase due to the influence of, for example, fading in the transmission path, therefore the transmission path is estimated by using the phase difference from each symbol received one modulation period before as a phase value of the received signal using each symbol received one modulation time before as a reference. The means for finding the phase of the received signal by this transmission path estimation is generally referred to as differential demodulation. The differential demodulation is carried out in the differential demodulation circuit 116. The thus extracted received symbols carrying information modulated in the phase component are returned to the original order of symbols by the frequency deinterleave circuit 115, then the symbol stream of the intended channel is extracted by the channel selection circuit 114.
The output channel stream from the channel selection circuit 114 is input to the bit extraction circuit 113. The bit extraction circuit 113 digitally demodulates the symbols of each sub-carrier to extract, for example, the received encoded bit stream for the QPSK modulated symbols. The time deinterleave circuit 112 returns the received encoded bit stream to the arrangement of the encoded bit stream of the original order by the time deinterleaving in the frame. Further, this is decoded for correcting errors by the channel decoder 111, whereby the information bit stream of the intended channel is obtained.
In a communication system comprising such a transmitting and receiving apparatus, the arrangement of symbols to be transmitted and received by the frequency axis and the time axis can be expressed as shown in FIG. 24. FIG. 24 shows the state of the symbols in the sub-carriers arranged on the frequency axis being differentially modulated with the symbols transmitted one modulation time before at the related frequency. This differential modulation is not closed in the channel. The differential modulation is carried out with the symbols of other channels.
In a communication system for transmitting multiplexed channels using the OFDM modulation method as previously proposed, the symbols of the intended channel are extracted after the transmission path is estimated for all channels together. Further, the symbols of the other channels are also necessary for extracting one channel worth of information, so by employing such a data structure isolation between channels is not possible in the modulation method and the transmission path estimation method. Since the above-mentioned DAB system is a broadcasting system and each channel is usually transmitting a signal constantly, isolation between channels in the modulation method and the transmission path estimation method has been considered.
Where handling packet transmission traffic, however, the channels are not always constantly transmitting and receiving signals, therefore with the above system configuration, modulation and demodulation become impossible, so it becomes necessary to perform the modulation and demodulation and the transmission path estimation in a closed state for every channels, that is, for every packet. Further, in general packet transmission traffic, the amount of the information to be transmitted at one time the amount of information per packet largely fluctuates from several tens of bytes to about several tens of kilobytes, for example. When handling such traffic, if modulation and demodulation are carried out by the known method the following disadvantages occur.
When differential phase modulation is applied as in the DAB system, the symbols transmitted one modulation period before are utilized as reference symbols for estimating the transmission path, therefore even in a case where desiring to transmit information which can be handled by the number of symbols in one modulation period, the transmission and the reception of two modulation periods worth of symbols including the reference become necessary. This is clearly wasteful from the viewpoint of the effective utilization of the transmission path bandwidth. In such a case, it is advisable to apply another method of estimation of the transmission path.
On the other hand, when considering the case where it is desired to transmit and receive a large volume of information, it is generally known that if the transmission path is estimated by the differential modulation used in the communication system described above, the required Eb/No, where Eb: energy per bit received by the receiving apparatus, No: received noise, Eb/No is the value expressing an S/N ratio per bit of the received data on the reception side, deteriorates by about 3 dB in comparison with the case where the estimation of the transmission path is carried out perfectly. When desiring to transmit and receive a large volume of information, transmitting symbols for estimating the transmission path in addition to the symbols modulated in accordance with information, precisely estimating the transmission path, and demodulating gives a lower total required Eb/No and enables signal transmission with a better efficiency. In this case, since transmission of the symbols for estimating the transmission path becomes necessary, the bandwidth is excessively used, but when the amount of the information to be transmitted is sufficiently large in comparison with the symbols for estimating the transmission path, resources are not wastefully used from the viewpoint of the required Eb/No. Further, if the encoding rate is raised by the amount of the lowering of the required Eb/No in order to provide exactly the bandwidth for the transmission of the symbols for the estimation of the transmission path, the bandwidth will not be excessively used.
In this way, for example, where information is transmitted in a burst-like manner and the amount of information to be transmitted per time fluctuates in a large dynamic range, as in packet transmission traffic, isolation is desirably taken in the modulation method and the transmission path estimation method for every channel. Further, preferably a different transmission path estimation method is used for every series of transmission information. The communication system of the prior art, however, has not given sufficient consideration to this.